This invention relates generally to semiconductor devices and their production, and more specifically to semiconductor devices with polycrystalline silicon electrodes and to a method for their fabrication.
Many semiconductor devices utilize doped polycrystalline silicon as an electrode. For example, in an insulated gate field effect transistor a doped polycrystalline silicon gate electrode is formed overlying a thin gate insulator. The polycrysalline silicon is heavily doped with conductivity determining impurities to increase the conductivity of the gate electrode. These conductivity determining impurities can seriously effect the integrity of the underlying gate insulator, especially in devices in which the gate insulator is only a few tens of nanometers in thickness.
The proper operation of the device requires a high conductivity electrode, but the dopant impurities used to achieve the high conductivity must be kept from diffusing into and through the underlying insulator to avoid shifts in threshold voltage and/or degradation of reliability of the gate insulator. This problem is espcially acute in the case of boron doping in the presence of a hydrogen containing ambient. Polycrystalline silicon, as it is usually deposited, is characterized by grain boundaries between individual columnar polycrystalline grains. These grain boundaries provide an easy path for the diffusing of dopant impurities because diffusion through the grain boundaries is much more rapid than through the silicon itself at typical processing temperatures. The problem of maintaining insulator integrity while achieving a high dopant concentration and high conductivity in the polycrystalline silicon is thus made worse by the enhanced diffusion of impurities along grain boundaries.
One solution which has been used to mitigate the degradation of the thin dielectric caused by segregation of dopant impurities at the polycrystalline silicon-insulator interface or in the insulator itself, has been to include a buried titanium nitride layer between two separate layers of polycrystalline silicon during fabrication of the gate electrode. Although this technique is effective for mitigating the degradation, it is an expensive solution which requires additional and costly processing steps.
Accordingly, a need existed for an improved method and structure for simultaneously achieving high conductivity of the polycrystalline silicon electrode and high reliability of the underlying insulator.
It is therefore an object of this invention to provide an improved semiconductor device.
It is a further object of this invention to provide an improved polycrystalline silicon electrode incorporating a barrier to dopant diffusion.
It is still a further object of this invention to provide an improved process for the fabrication of polycrystalline silicont electrodes and semiconductor devices.